It is well known to apply a wear alloy coating to a substrate material to improve its resistance to abrasion, galling, hammering, moisture erosion, solid particle erosion or other types of wear. “Hard facing” is defined in Materials Handbook, Ninth Edition, Volume 3, published by The American Society of Metals, on pages 563-567, as “the process of applying, by welding, plasma spraying or flame plating, a layer, edge or point of wear-resistant metal onto a metal part to increase its resistance to abrasion, erosion, galling, hammering or other form of wear.” Nonferrous alloys are also used for wear applications, both as wrought parts and as coatings, as discussed on pages 589-594 of the same Materials Handbook. The term “wear alloy” as used herein is meant to include both the hard facing materials discussed on pages 563-567 and the nonferrous alloys discussed on pages 589-594 of the Material Handbook.
Wear alloys are frequently used in applications where systematic lubrication against abrasion is not feasible or is inadequate to give a desired service life to a component. New parts may be provided with a wear alloy coating in selected areas and worn parts may be refaced multiple times before replacement of the entire part becomes necessary, thereby reducing the lifetime cost of the part.
Hard facing materials are classified in Materials Handbook into five major groups defined primarily according to total alloy content (elements other than iron). Generally, as the group number increases from Group 1 to Group 5, the alloy content, wear resistance and cost will all increase. Groups 1, 2 and 3 hard facing materials are ferrous materials generally contain a total alloy content of less than 50%. Group 4 materials contain from 50-100% alloy content, typically nickel-based and cobalt-based alloys with alloying elements of nickel, chrome, cobalt, boron and tungsten. Group 5 materials consist of hard granules of carbide distributed in a metal matrix. The carbide may be tungsten carbide, titanium carbide, chromium carbide or tantalum carbide. The metal matrix may be a ductile material such as iron, cobalt or nickel. Carbide based wear resistant materials are often used in applications of severe low stress abrasion where cutting edge retention is needed. Low stress wear resistance is an important component of a carbide material's performance. Some carbide systems, such as those with chromium carbide, also afford significant high temperature oxidation/corrosion resistance while retaining excellent wear resistance properties.
Nonferrous wear alloys may be wrought cobalt-base alloys (such as commercial brands sold under the names of Stellite 6B, Stellite 6K, Haynes 25 and Tribaloy T-400), beryllium-copper alloys (for example C17200) and certain aluminum bronzes (C60800, C61300 and C61400 soft ductile alloys and very hard proprietary die alloys).
Welding, brazing and flame spraying techniques have been used to apply wear alloy coatings. Brazed materials are limited in their potential uses by the melting temperature of the braze alloy. A welded or flame sprayed wear alloy coating may be subject to cracking upon its application due to the shrinkage cracking of these relatively brittle coating materials. Furthermore, the heat input during the application of a wear alloy coating may cause warping of a relatively thin substrate member such as a turbine blade. The heat input from the application of a wear alloy coating may melt or otherwise metallurgical degrade properties of an underlying single crystal or directionally stabilized substrate material or a proximate brazed joint.
Dilution is the interalloying of the wear alloy and the base metal, and it is usually expressed as the percentage of base metal in the deposited wear alloy. A dilution of 10% means that the deposit contains 10% base metal and 90% wear alloy. As dilution increases, the hardness, wear resistance and other desirable properties of the deposit are reduced. The amount of dilution may vary depending upon the deposition process being used and the thickness of the coating. One known technique used to control the amount of dilution it to deposit a buffer layer between the base metal and the wear alloy.
For applications requiring a thick layer of hard face coating material, several coating layers may be used. However, highly alloyed deposits are likely to spall if applied to a thickness of more than 6 mm (¼ inch) as a result of interfaces created within the coating by splat boundaries between sprayed layers or brittle phases between welded layers.